Ammeter Problems

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Apr 21, 2003 | 04:04 PM

Bern_F150_4x4

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From: Maryland

Ammeter Problems

Ok, I have read a lot of electrical system problem posts and most of the time the word is to ditch the ammeter for a voltmeter.

My charging system works fine,
I just want my gauge to work!

Here is the scenario:

When I bought the truck the gauge didn't work.
I have collected many gauges from the yard and none of them have "worked".
Just for the hell of it, I hooked one of the gauges up in series with a small strobe light and as expected, when the strobe blinked the gauge needle deflected.
I'm finding it hard to believe that any of my gauges are bad. I feel I have a wiring problem.

So my question is:
How do I trouble-shoot this problem?
The factory shop manual is not very much help.

I have read the voltages at the instrument cluster connection, but not knowing what to expect, didn't know how to interpret them.
I believe one pin was at battery voltage (engine running) and the other was ~7 Volts. All wiring under the hood looks in good shape.

Thanks for any help resolving this matter!

Apr 21, 2003 | 07:16 PM

Franklin2

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From: Virginia

Ammeter Problems

The factory guages use a shunt. The shunt is a small calibrated resistance that produces a tiny voltage drop. The voltage drop varies with how much current is going through the shunt. The factory amp guage is really a volt guage designed to read the small voltage coming from the shunt. Yes, all the guages are probably good. And your wiring is probably ok too. Not one of Ford's better ideas. But it does work better than the Dodge system, which has a real amp guage, but the wiring burns up all the time.

Apr 21, 2003 | 08:04 PM

Bern_F150_4x4

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From: Maryland

Ammeter Problems

Dave thanks for the fast reply.

I am familliar with the basic operation of the gauge/charging circuit.

Is there a common failure with the gauge/circuit in these trucks?

Where do you recomend beginning the troubleshooting?

Where the heck is the shunt?

I would really like to get it working again.

Apr 22, 2003 | 08:04 PM

steve83

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From: Memphis, TN

Ammeter Problems

The shunt is either in the alternator harness or in the engine compartment harness nearby. I deleted it in my truck when I switched to EFI, the ALT light, and the volt gauge. Follow the 2 wires from the AMP gauge and the shunt is the heavy wire between them. The Haynes diagram indicates it, but it's a little easier to understand if you look at the '81-83 diagram. In that diagram, it's the Bk/Or SHUNT just R of the voltage regulator.

Last edited by steve83; Apr 22, 2003 at 08:10 PM.

Apr 30, 2003 | 01:48 PM

Bern_F150_4x4

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From: Maryland

Ammeter Problems

Thanks a bunch for all of the advice! Between my post and similar ones, I was able to gain enough knowledge to perform the troubleshooting required to find the problem.

And I found it, which turned out to be a corroded wire (red w/yellow) on the battery side of the shunt. The wire joins a connection that includes a yellow wire from the regulator and the fuseable link to the solenoid / Relay (bat+).

The corrosion was hard to spot. What I assume happened is that someone had used a puncture-type testlight on the wire which damaged it by allowing battery acid etc to enter the wire and the rest is history.

Thanks again, you guys are the best!